XML 51 R40.htm IDEA: XBRL DOCUMENT
v3.10.0.1

Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract] |  |  |  |
Net realized investment (losses) gains, pre-tax | $ (10.2) | $ 6.8 | $ (13.9) | $ 20.8
Net unrealized investment gains, pre-tax | 76.3 | 25.7 | 36.8 | 81.7
Net realized and unrealized investment gains, pre-tax | 66.1 | 32.5 | 22.9 | 102.5
Income tax expense attributable to net realized and unrealized investment gains | (9.7) | (3.9) | (5.9) | (9.5)
Net realized and unrealized investment gains, after-tax | $ 56.4 | $ 28.6 | $ 17.0 | $ 93.0

X
- References No definition available. + Details
Name: | us-gaap_InvestmentsDebtAndEquitySecuritiesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The sum of the realized and unrealized net gain (loss) on trading securities during the period. + References No definition available. + Details
Name: | us-gaap_NetRealizedOrUnrealizedGainLossOnTradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Net Realized and Unrealized Gain (Loss) on Trading Securities, Net of Tax + References No definition available. + Details
Name: | wtm_NetRealizedandUnrealizedGainLossonTradingSecuritiesNetofTax
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period. + References No definition available. + Details
Name: | wtm_NetUnrealizedGainLossandForeignCurrencyGainLossonTradingSecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Tax on net realized and unrealized gains (loss), trading securities + References No definition available. + Details
Name: | wtm_Taxonnetrealizedandunrealizedgainslosstradingsecurities
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Trading Securities, Realized Gain (Loss), Total + References No definition available. + Details
Name: | wtm_TradingSecuritiesRealizedGainLossTotal
Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration